Field of the Invention
The present invention relates to a temperature-dependent switch comprising a temperature-dependent switching mechanism, a housing accommodating the switching mechanism, two first connections provided on the switch, between which first connections the switching mechanism makes or opens an electrically conductive connection depending on the temperature of said switching mechanism, and comprising a heating resistor, which is arranged on an outside of the housing and is connected electrically in series with the two first connections.
Related Prior Art
Such a switch is known from DE 43 36 564 C2.
The known switch is embodied in the form of an encapsulated switch comprising a two-part, current-conducting metal housing, as is known, for example, from DE 21 21 802 A or DE 196 23 570 C2 as well. The encapsulated switch is arranged on a mounting plate consisting of ceramic, on which mounting plate a thick-film resistor is arranged between conductor tracks, which thick-film resistor is electrically connected at one of its ends to the conductive lower part of the encapsulated switch. The other end of this heating resistor is connected to one of the conductor tracks, which acts as the soldering surface to which a first connection strand is soldered. The second connection strand is electrically soldered to the conductive cover part of the encapsulated switch.
The lower part of the switch rests with its outer base on the heating resistor. The thick-film resistor can in this case be covered by an insulating layer. The switch is intended to be soldered to a lateral conductor track on the mounting plate, wherein no mention is made in this document of how the soldering is intended to take place. As a result, a linear cohesive contact is produced between the lower part and the conductor track acting as soldering surface.
It is not only problematic to produce this connection, additionally it also has insufficient mechanical stability, for which reason the document discloses that heat-shrink tubing is shrunk onto the switch and the mounting plate together, and the two connection strands protrude out of said heat-shrink tubing laterally. As a result, the switch and the mounting plate are additionally mechanically fixed to one another specification.
Such temperature-dependent switches are used in a known manner for protecting electrical appliances from overheating. For this purpose, the switch is connected electrically in series with the appliance to be protected via its two first connections and is arranged mechanically on the appliance in such a way that it is thermally connected thereto.
In the embodiment of a switch in accordance with DE 196 23 570 C2, a temperature-dependent switching mechanism comprising a spring disc, a bimetallic snap-action disc and a movable contact part is arranged in the housing, which movable contact part is in bearing contact with a stationary contact part on the inside on the upper part in the closed state of the switch, which stationary contact part is through-plated towards the outside to a first connection on the upper part. The conductive lower part acts as further first connection.
The operating current of the appliance to be protected thus flows through the two contact parts and the spring disc into the lower part.
The switch known from DE 43 36 564 C2, by virtue of the heating resistor, is equipped with a current-dependent switching function, for which purpose the heating resistor is connected permanently electrically in series with the first connections. The operating current of the appliance to be protected therefore flows continuously through this heating resistor, which can be dimensioned such that, in the event of a specific operating current being exceeded, it ensures that the bimetallic snap-action disc is heated to a temperature above its response temperature, with the result that the switch opens at a high operating current even before the appliance to be protected has heated to an impermissible extent.
Below the response temperature of the bimetallic snap-action disc, the circuit is closed and the appliance to be protected is supplied with current via the switch. If the temperature increases beyond a permissible value, either as a result of an excessively high operating current or as a result of an excessively heated appliance to be protected, the bimetallic snap-action disc deforms, as a result of which the switch is opened and the supply to the appliance to be protected is interrupted.
The now de-energized appliance can then cool down again. In the process, the switch which is thermally coupled to the appliance also cools down again, which switch thereupon closes again automatically. While such a switching response may be quite sensible for protecting a hairdryer, for example, this is not always desirable when the appliance to be protected should not automatically switch on again after shutdown in order to avoid damage. This applies, for example, to electric motors which are used as drive assemblies.
In known temperature-dependent switches, therefore, a so-called self-holding resistor is often provided, which is electrically in parallel with the first connections; see, for example, DE 195 14 853 A1. The self-holding resistor is electrically in series with the appliance to be protected when the switch is open, and now only a harmless residual current flows through the appliance owing to the resistance value of the self-holding resistor. This residual current is sufficient, however, to heat up the self-holding resistor to such an extent that it emits heat, which keeps the bimetallic snap-action disc above its switching temperature.
As a deviation from the embodiment of the switch in accordance with DE 196 23 570 C2, the temperature-dependent switching mechanism can also comprise only a bimetallic snap-action disc, which supports the movable contact part and therefore conducts the operating current.
The switching mechanism can also comprise a bimetallic spring tongue, as is described in DE 198 16 807 A1. This bimetallic spring tongue supports, at its free end, a movable contact part, which interacts with a stationary counter contact. The stationary counter contact is electrically connected to one of the first connections, wherein the other first connection is electrically connected to the clamped-in end of the bimetallic spring tongue. The bimetallic spring tongue in this case conducts the operating current of the electrical appliance to be protected.
If the temperature-dependent switch is intended to conduct particularly high currents, a current transfer element in the form of a contact bridge or a contact plate is often used, which is moved by a spring part and supports two contact parts, which interact with two stationary counter contacts.
In this way, the operating current of the appliance to be protected flows from the first counter contact via the first contact part into the contact plate, through said contact plate to the second contact part and from there into the second counter contact. The spring part is thus de-energized. It is also known to use the spring part itself, i.e. for example a bimetallic snap-action disc or a spring snap-action disc operating against a bimetallic part as contact bridge.
In particular when the known switches are used for protecting high-power motors, they need to be able to be subjected to a very high mechanical load owing to the severe vibrations occurring during operation and in particular during run-up of the motors.
In addition, the switches need to be capable of protecting the motors reliably both during critical operation at the maximum permissible power and in the case of a locked rotor. In order to check whether the switch also achieves this, two tests are implemented in a conventional manner.
In the so-called Heating Test, the motor is operated at maximum power, wherein neither the current flow through the switch nor the heat transmitted in the process from the motor to the switch should open the switch.
In the so-called Locked Rotor Test, on the other hand, the motor is connected to the operating voltage when the rotor is locked, which results in an operating current flowing through the motor which is three to five times higher than the conventional operating current.
This high current naturally also results in heating of the motor and therefore in a temperature increase at the switch.
However, this heating-up takes place so slowly that the motor may already be irreversibly destroyed before the switch responds as a result of the increase in the motor temperature. Therefore, in this test a heating resistor needs to ensure that the switch opens very quickly.
Even in the case of suitable matching between the response temperature of the bimetallic snap-action disc and the resistance value of the heating resistor on its own, these two contradictory conditions cannot be met in the above-described known switches, however.
These values could indeed be set such that the maximum permissible operating current does not result in the heating resistor heating the bimetallic snap-action disc to a temperature above its switching temperature but such that this only takes place as a result of the markedly higher current in the case of a locked rotor.
Secondly, the response temperature of the bimetallic snap-action disc could be selected such that it is above the temperature which is assumed by the motor during operation at maximum permissible power and which is transferred to the switch, but below the temperature to which the bimetallic snap-action disc is heated by the heating resistor when the current flows through it when the rotor is locked.
The switching response set in this way is only achieved during the steady-state operation, however, i.e. if sufficient time has lapsed in order for the switch to open, either when the temperature of the motor is too high or else when the current is too high. For the protection of a high-power motor, however, it is also necessary for the switch to respond extremely quickly, in particular in the case of a locked rotor.
This requires very good thermal coupling of the heating resistor to the switch in order that a change in the temperature of the heating resistor is transmitted to the bimetallic snap-action disc in the shortest possible time.
In addition to good thermal coupling, the switch also needs to perform the required number of switching cycles, which should be at least 3000 in the case of typical requirements, such as have been described above. For relatively low operating currents up to approximately 4 amperes and switching temperatures of approximately 160° C., the known switches meet these requirements as well.
In the case of relatively high operating currents of 10 amperes or more, the number of switching cycles is markedly reduced, however, because the temperature change at the soldered joints between the lower part and the mounting plate result in the soldered joints being damaged as a result of fatigue failure after approximately 1000 switching cycles to such a great extent that the current flow is interrupted and the switch is not operational.
DE 10 2011 016 133 B4 therefore proposes, in contrast to the embodiment disclosed in DE 43 36 564 C2 cited at the outset, to solder the bottom of the switch and not only the lateral transition between the bottom and the side wall onto the mount flat.
The bottom of the switch is used here for two purposes, firstly it rests over the full area on the heating resistor, and secondly it is held flat on the soldering surface by means of a cohesive joint. By virtue of this flat cohesive joint, next to the heating resistor, a very good thermal connection of the switch to the heating resistor results, wherein the switch is not only fixed mechanically very stably to the mounting plate in this way, but this type of fixing also results in the desired thermal coupling.
With this known switch, the design described to this extent entails the risk that, when handled improperly, a force is exerted on the mounting plate and/or the housing, which force results in the soldered joints breaking or at least being weakened.